Porous solids with tailored pore characteristics have attracted considerable attention because of their novel optical, catalytic, sensing, and electrochemical properties. Examples having a generally periodic pore structure include photonic crystals and photonic bandgap materials. Examples which may not require a periodic pore structure include separation membranes, mesoporous molecular sieves, and three-dimensionally (3D) porous metals. Porous metals in particular are widely used in energy conversion or storage devices, as filters, as catalyst supports, as electromagnetic wave absorbers, and as biomedical scaffold materials.
Porous conductive current collectors are used in commercially available rechargeable batteries, such as nickel metal hydride (NiMH) batteries. Most commercial NiMH battery cathodes employ sintered plaque or nickel foam as the current collectors. The performance of the nickel foam is limited, however, by large pore sizes and a broad pore size distribution. When the foam is impregnated with nickel hydroxide, the charge storage media, protons and electrons have to travel a long distance between the nickel metal and the Ni(OH)2/electrolyte interface. Because the reduced state of the charge storage material is nearly an insulator, discharge becomes difficult due to increasing resistance along the ion and electron transfer paths. Thus, commercial batteries lose considerable capacity when discharged at a high C rate (nC rate is defined as the full use of the battery capacity in 1/n hour). For example, when discharged at a 35 C rate the capacity of a NiMH battery with a commercially available nickel foam is only 1.7% of its slow discharge capacity. Charge is the reverse process of discharge. However, due to the asymmetric characteristics, charge is more difficult than discharge at the same rate. Therefore, shortened ion/electron pathways may have a significant effect on improving the charging capability of batteries.